Cardiovascular diseases (CVD) remain the leading cause of death in modern society in developed nations. The number of older adults in the developed world is expected to at least double by 2050, and this is associated with projections of marked increases in CVD burden.
Aging increases the risk of CVD largely due to the dysfunction of the arteries, namely endothelial dysfunction and large elastic artery stiffness. Vascular endothelial dysfunction is primarily assessed via endothelium-dependent dilation (EDD) and is impaired largely due to increased superoxide (O2−) production. Increased O2− reduces the bioavailability of the potent vasodilator and vasoprotective molecule nitric oxide (NO). Increased aortic stiffness, in particular, reduces the ability to buffer increases in pressure produced by systolic ejection of blood into the large elastic arteries with each cardiac contraction. This increases systolic blood pressure and arterial pulse pressure (the difference between systolic and diastolic blood pressure), as well as the “pulsatility” of blood flow, which is transmitted to the microvasculature of vulnerable high-flow organs such as the brain and kidney, causing end-organ damage and other pathophysiological effects. Similarly, increased arterial stiffness has been linked to endothelial dysfunction and is now recognized as the major independent risk factor for age-associated CVD. Therefore, there is an urgent need to develop treatments that reduce the risk of CVD with aging.
Without being bound by theory, the mechanisms by which arteries stiffen with age are not completely understood, but are thought to include changes in the composition of structural proteins within the arterial wall. Collagen (type I) is the primary load-bearing protein in the arterial wall, and its abundance is increased with advancing age. In contrast, elastin, the main structural protein conferring elasticity, is reduced in old arteries. It has previously been shown in mice (Fleenor B. S. et al. 2012a, Aging Cell. 11, 269-276) and cultured aortic fibroblasts (Fleenor B. S. et al. 2010, J Physiol. 588, 3971-3982) that oxidative stress contributes to some or all of the age-associated structural changes seen within the arteries.
Two key antecedents and independent predictors of clinical CVD in older adults are thought to include vascular endothelial dysfunction, assessed by endothelium-dependent dilation (EDD), and large elastic artery stiffness, measured by aortic pulse wave velocity (aPWV). A common mechanism that contributes to both vascular endothelial dysfunction and large elastic artery stiffness with aging is believed to involve excessive superoxide-associated vascular oxidative stress (Seals D. R. et al. 2011, Clin Sci 120, 357-375; Fleenor B. S. et al. 2012a, Aging Cell. 11, 269-276; Bachschmid M. M. et al. 2013, Ann Med. 45, 17-36). Increased vascular production of superoxide occurs with aging and reduces the bioavailability of the vasoprotective and vasodilatory molecule nitric oxide (NO), while also causing alterations in major structural proteins (collagen and elastin) in the large elastic arteries (i.e., the aorta and carotid arteries). These changes contribute directly to age-related endothelial dysfunction and increased arterial stiffness. As such, treatments that reduce the excessive superoxide production in aging arteries hold the potential for improving age-associated vascular dysfunction.
It is recognized that there is an association between endothelial dysfunction and a decline in cognitive and motor (physical) function during both normal aging and in age-associated disease states. It is further recognized that endothelial function plays a role in the systemic regulation of metabolism, blood fluidity, tissue perfusion, immune function and enhancement of longevity.
It has been previously shown that lifelong caloric restriction (CR), as well as short-term CR in old animals, prevents or reverses endothelial dysfunction and large elastic artery stiffening by reducing superoxide production, increasing NO bioavailability and modifying structural proteins (Rippe C. et al. 2010 Aging Cell. 9, 304-312; Donato A. J. et al. 2013, Aging Cell. 12, 772-783). However, because adherence to CR is not practical for most humans, there is growing interest in pharmacological therapies that may induce the benefits of CR.
Sirtuins are a class of enzyme proteins that possess deacylase activity, including deacetylase, desuccinylase, demalonylase, demyristoylase and depalmitoylase activity, some of the sirtuins (for example, SIRT6) also possess mono-ribosyltransferase activity. The expression and activity of sirtuin enzymes is reduced with advancing age. There are 7 mammalian sirtuins (SIRT 1-7) that correspond to the yeast Sir2 (silent mating-type information regulation). Sirtuins also possess nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase activity. Mammalian SIRT1, one of seven members in the sirtuin family of protein deacetylases/deacylases, is a nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase that acts as a metabolic energy sensor implicated in several of the beneficial effects of CR, including reduced oxidative stress (Boily G. et al. 2008, PLoS One. 3, e1759; Merksamer P. I. et al. 2013, Aging (Albany N.Y.). 5, 144-150). Enhancing NAD+ biosynthesis with NAD+ precursors, such as NMN and nicotinamide riboside (NR), increases the activity of the NAD+-dependent deacetylase SIRT1 (Imai S. 2010, Pharmacol Res. 62, 42-47; Satoh A. et al. 2011, Handb Exp Pharmacol. 206, 125-162; Canto C. et al. 2012, Cell Metab. 15, 838-847).
With advancing age, there is a reduction of the expression and activity of sirtuins in mammals and humans. Because of this, sirtuins are believed to influence many aging processes. SIRT1 expression in endothelial cells is positively associated with EDD in young and older adults (Donato A. J. et al., 2011 J. Physiol. 589, 4545-4554), implying that SIRT1 may influence vascular function in humans. Previous studies show that reduced SIRT1 expression and activity is a key mechanism mediating impaired EDD in aging arteries (Rippe C. et al. 2010, Aging Cell. 9, 304-312; Donato A. J. et al. 2011, J Physiol. 589, 4545-4554; Gano L. B. et al. 2014, Am J Physiol Heart Circ Physiol. 307, H1754-1763), and recent findings indicate that pharmacological activation of SIRT1 with the compound SRT1720 improves EDD in old mice in part by reducing oxidative stress (Gano L. B. et al. 2014, Am J Physiol Heart Circ Physiol. 307, H1754-1763). Oxidative stress has been shown to play an important role in the development of vascular endothelial dysfunction and large elastic artery stiffness associated with increasing age (Lakatta E. G. & Levy, 2003 Heart Fail Rev. 7, 29-49; Seals et al. 2011, Clin Sci 120, 357-375). Oxidative stress in the vasculature leads to a decrease in NO bioavailability, thus causing endothelial dysfunction and stiffening of the large elastic arteries. Superoxide reacts with NO, forming peroxynitrite (ONOO−), which reduces the bioavailability of NO; this results in less bioavailable NO to contribute to vasodilation. Furthermore, ONOO− oxidizes tyrosine residues on proteins post-translationally producing nitrotyrosine, one key marker of oxidative stress.
It has been established that oxidative stress and inflammation are intimately connected (Csiszar A. et al. 2008, J Appl Physiol. 105, 1333-1341; Ungvari Z. et al. 2010, J Gerontol A Biol Sci Med Sci. 65, 1028-1041) and SIRT1 has been found to modulate the activity of nuclear factor kappa B (NFκB) and tumor necrosis factor alpha (TNFα) (Yoshino J et al., 2011, Cell Metab. 14, 528-536), both of which are master regulators of the inflammatory process. The p65 subunit of NFκB is a major target of SIRT1, and is deacetylated in response to SIRT1 activation.
NAD+ bioavailability also decreases with age in various mammalian tissues, and restoring NAD+ levels has been shown to ameliorate high-fat diet- and age-induced Type 2 Diabetes in mice while restoring gene expression related to oxidative stress and inflammation to that of a healthy, non-diabetic mouse, partly through SIRT1 activation.